Over the past several years, a significant increase in recording density in thin-film media magnetic recording disks has been achieved, and there is a continuing effort to increase recording density further. A number of magnetic properties in a thin-film disk are important to achieving high recording density. These include high coercivity, high remanence, and low flying height, that is, a close proximity of the read/write heads to the disk surface. Such proximity decreases overlap of voltage signals in adjacent magnetic domains in the disk, thus permitting an increase in recording density and optimum read-write performance.
To reduce flying height, and to improve wear resistance, it is desirable for the surface of the disk to be as smooth as possible. However, prolonged contact of a read-write head with a very smooth disk surface, i.e. when the head is not in use, can lead to the problem of "stiction", wherein the two closely matching surfaces stick to each other, causing possible damage during start/stop cycles. Manufacture of such disks thus typically includes a texturing step, to create a roughened substrate surface, characterized by submicron surface irregularities. The roughened surface reduces stiction between the disk and head by reducing surface contact between the two, particularly for start/stop cycles. Such texturing is often carried out only on a nondata region near the inner opening of the disk, where the read/write head is "parked" when the disk is not operating.
Such texturing can lead to an undesirable increase in flying height if there is too much variation in the surface irregularities created. The best (lowest) flying head distances which have been achieved with plated metal-substrate disks polished by sanding or abrasion, as described above, is about 6 microinches (mils). Laser texturing of such magnetic media, in which textured spots on the surface are produced by controlled melting and solidifying at closely spaced locations on the plated alloy, has been reported to produce flying head distances in the range of one microinch or less (Ranjan), a clear improvement over abrasion-based methods.
For maximum uniformity in laser texturing, however, it is important to maintain a constant focus of the laser beam at the surface of the disk as the beam is moved from one spot on the disk to another. Certain variables exist in previously described methods which could lead to variations in the path length between the laser source and the disk surface, resulting in variable focus at the disk surface, and a resulting lack of uniformity in the size and depth of the textured sites. For example, in texturing a disk, it is generally desirable to produce a pattern of textured spots over a central surface region of the disk. This is achieved in the reported methods by rotating the disk on a spindle, in conjunction with radial stepping of the laser or the disk. If any oscillatory motion occurs during rotation of the disk, or if the precise distance between the laser and disk surface is not maintained during radial stepping, variations in depth of texturing greater than the order of a microinch are likely to result.
Thus, it is desirable to provide a laser texturing apparatus and method which allows precise focus of the laser beam to be maintained at the disk surface as the beam is directed to various points on the surface.